1. Field of The Invention
This invention relates to novel silacyclohexane compounds and the preparation thereof. The invention also relates to liquid crystal compositions comprising the silacyclohexane compound or compounds and to devices comprising the compositions.
2. Description of the Prior Art
The liquid crystal display devices utilize optical anisotropy and dielectric anisotropy of liquid crystal substances. Depending on the mode of display, different types of display systems are known including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence type (SBE type), a dynamic scattering type (DS type), a guest/host type, a type of deformation of aligned phase (DAP type), a polymer dispersion type (PD type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid crystal substances used in these liquid crystal devices depend, more or less, on the type of display, it is commonly required that the liquid crystal substances have a wide range of temperatures working as a liquid crystal and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in cells.
Liquid substances which can satisfy all the requirements have never been known when used singly. In practice, several to ten and several liquid compounds and/or latent liquid crystal compounds are used in the form of a mixture. To this end, it is important that constituent components be readily compatible with one another.
Liquid crystal substances usable as these constituent components can be broadly classified into the following groups depending on the function thereof.
(1) Compounds contributing to the reduction of viscosity and lowering of melting point of mixed liquid crystal compositions.
(2) Compounds mainly controlling electrooptic functions of liquid crystal compositions.
(3) Compounds contributing to the increase in transparency of mixed liquid crystal compositions.
(4) Compounds contributing to the control of anisotropy in refractive index of mixed liquid crystal compositions.
(5) Compounds controlling the color display and alignment of mixed liquid crystal compositions.
The compounds classified as (1) above are known including those compounds having a cyclohexane ring-methylene-oxy-benzene ring structure of the following formula having an ether bond ##STR5## wherein R represents an alkyl group having from 1 to 8 carbon atoms as set out in Japanese Patent Publication No, 4-60976.
Moreover, those compounds having cyclohexane ring-methylene-oxy-benzene ring structured of the following formulas having an ether bond are known as having a relative high melting point or T.sub.NI (nematic-isotropic phase transition temperature) and a wide temperature range wherein the nematic phase is maintained ##STR6## wherein R represents an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 63-2246. ##STR7## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 63-46740.
As the liquid crystal display devices recently have wider utility, the characteristic properties required for the liquid crystal materials become severer. Especially, it is strongly required in the field of portable devices for multi media that batteries be small in size and thus, the power consumption be low. Hence, liquid crystals capable of driving at low voltage are correspondingly required. To realize the low drive potential, liquid crystal materials should have a great dielectric anisotropy (.DELTA..epsilon.).
On the other hand, as liquid crystal display devices have wider utility in various fields, the characteristic properties required for liquid crystal materials become severer. In fact, liquid crystal materials now demanded should have an improved low temperature performance, a wider working temperature satisfying on-vehicle needs and a lower drive voltage on comparison with existing liquid crystal materials or compositions.